--> --> Imaging the Faults using Double Difference Relocation in Azle-Reno, TX

AAPG Southwest Section Annual Convention

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Imaging the Faults using Double Difference Relocation in Azle-Reno, TX

Abstract

The Azle-Reno earthquake sequence, north and west of Fort Worth, produced the first felt event in November 2013 and continued to generate significant numbers of felt events through January 2014, with magnitudes reaching M 3.6. Seismicity continued after January 2014, there were no widely felt events again until June 2017. Wastewater injection in the region began in early 2009, well before earthquakes, and has had periods of high and low volumes. Previous study showed that the Azle-Reno earthquakes occurred on two segments of steep normal faults in the crystalline basement-a shallow antithetic fault and main fault that enters deeper sections of the basement-and are linked to disposal and production operations that effect pressure within the Ellenburger injection unit and underlying basement. The presentation updates and extends initial published studies. We present the high-resolution earthquake relocations using double difference (DD) relocation technique for the complete time period of SMU local seismic network in Azle-Reno region, December 2013- present. Ambient noise tomography, on a data set collected from 131 1-component Nodal Seismic array deployed for 10 days near Azle, constrain the upper 100 meters of the 1D Vp - Vs profile used for location. Tomography results are combined with local geology, regional crustal structure, and well log data to make a 1D velocity model for high resolution earthquake studies. Differential arrival times for event pairs are derived from the catalog and via cross-correlation over a bandpass of 5-20 Hz for P-wave and 2-25 Hz for S- wave. The waveforms show a high degree of similarity within active fault segments and we constrain the geometry of a newly active segment of fault located north of the bulk of activity in late 2013 and early 2014. DD relocations provide a refined view of the depth distribution of earthquakes. We show that most large earthquakes occur in crystalline basement while swarms of small magnitude events occur in the Ellenburger, the primary wastewater injection unit, and in the basement. The comparison of the seismicity along the faults and wastewater injection data showed although there is high seismicity rate till 2015, the seismicity decreased while the injection rates continued at the same levels.